Abstract: When applying the Nernst equation to calculate the electromotive force of chemical power sources under non-standard conditions, it is essential to determine the activity of solid/liquid pure phase substances in the equation for proper simplification. Current textbooks on Physical Chemistry and Principles of Electrochemistry, however, fail to provide guidance on distinguishing between pure liquid and solid phases among reactants and products in battery reactions. This omission frequently leads to errors during students’ attempts to simplify the Nernst equation. To address this issue, this paper systematically examines four representative chemical power sources—lead-acid batteries, zinc-nickel batteries, lithium-ion batteries, and sodium-sulfur batteries—demonstrating their respective Nernst equation simplification methods. Furthermore, it establishes clear criteria for identifying the activity of pure solid or liquid phases within the equation.

Key words: Nernst equation, Reversible electrode, Chemical power source, Electromotive force of cell, Activity